1. Field of the Invention
The present invention relates to a method for recording image and an apparatus therefor, and a medium recorded by such an apparatus. More particularly, the invention relates to an image recording method to record an image on a recording medium by enabling a recording head to scan, and an apparatus therefor, and a medium recorded by such an apparatus.
2. Related Background Art
There is well known an image recording apparatus of the so-called serial scanning type wherein a recording head provided with a plurality of recording elements (exothermic resistive members, nozzles, and the like) is caused to scan for recording. FIG. 2 is a view illustrating a recording method of such a serial scanning type as this, in which a reference numeral 201 designates an ink jet head with an arrangement of plural nozzles 202. An image recording is performed per recording width d corresponding to the arrangement of the nozzles 202 of this ink jet head 201 while the ink jet head 201 is being scanned on a recording sheet 203 in the direction A. Thus, when a recording of the recording width d is terminated, the recording sheet 203 is shifted in the direction B for a length corresponding to the recording width d. Then, the ink jet head 201 is again caused to scan in the direction A to perform another image recording for the recording width d. The recording by such a serial scanning method as this has an advantage that an image data having a large image surface can be recorded by a small head. On the other hand, there is a disadvantage that should there be any nozzles that may be disabled to eject ink or may cause the positions of recorded dots to be displaced among the nozzles of the head 201, such a portion appears continuously in the direction A, which tends to create continuous white streaks. In order to compensate for a disadvantage of the kind, there is proposed a recording method by multi-scanning which will be described later.
FIG. 3A is a view illustrating such a multi-scanning as this.
An ink jet head 301 has twelve nozzles as designated by reference numerals 1-1 to 1-12. These nozzles can be divided into two portions indicated by reference marks X and Y. Here, the nozzles corresponding to the X portion are represented by 1-1 to 1-6 while the nozzles corresponding to the Y portion are represented by 1-7 to 1-12. At first, a recording by the X portion of the ink jet head 301 is performed with the initial scanning for recording on the portion of a recording sheet 203 at X′ (the recorded dots by this recording are represented by X1 to X6). Then, in continuation, the recording sheet 203 is shifted in the direction B in the sub-scanning direction by an amount d in order to record dots Y1 to Y6 (represented by Y′) using the Y portion of the head 301. By recording in this way, the dots recorded by the use of the same nozzles are not continuous in the direction A. Therefore, even if there are nozzles causing the positions of the recording dots to be displaced, the dots thus recorded do not appear continuously in the direction A; hence resulting in an advantage that the white streaks in the direction A are not remarkably noticeable.
Also, the recording density unevenness due to the irregularity of ink ejection amounts per nozzle is offset by the recording thus performed, and such unevenness is not remarkably noticeable, either. Also, if the recording duty of the ink jet head is high, the ink mist is accumulated in the vicinity of the orifice to hinder the ink ejection in some cases, but when the multi-scanning is performed, dots are thinned out to enable the number of ink ejections per unit period of time to be reduced; hence suppressing the generation of the mist. An advantage is brought about that the disabled ejection due to mist is reduced.
Nevertheless, there are still the images for which the ink ejection defects causing the white streaks, density unevenness, and mist cannot be prevented only by the foregoing two-time multi-scanning. For example, if a uniform pattern should be recorded, the white streaks and density unevenness become extremely conspicuous, and in some cases, not only the foregoing two-time multi-scanning, three- or four-time multi-scanning is also required.
Also, for an image requiring a high recording duty, it is insufficient to make the recording duty a half by the two-time multi-scanning. There are some cases where it is better to reduce the recording duty to a ⅓or ¼by the three- or four-time multi-scanning.
On the other hand, however, there is a problem that if the number of multi-scannings is increased, the recording period of time is prolonged that much.
FIG. 3B is a view illustrating another example of such a multi-scanning as this.
The nozzles 302 of the ink jet head 301 can be divided into three portions designated by reference marks X, Y, and Z. The portions include the nozzles X-1 to X-4, Y-1 to Y-4, and Z-1 to Z-4, respectively. At first, with the initial scanning, a recording is performed by the portion X of the ink jet head 301 for the portion of the recording sheet 203 at X′ (the dots formed by this recording are represented by X-1 to X-4). Then, the recording sheet 203 is shifted in the direction B by d in the sub-scanning direction, and dots Y-1 to Y-4 (represented by Y′) are recorded using the portion Y of the head 301. However, at this juncture, the portion X of the ink jet head 301 performs its recording in a position at X. Then, continuously, the recording sheet 203 is again shifted in the direction B by d for recording by the use of the portion Z of the ink jet head 301. At this juncture, as shown in FIG. 3B, the recording is performed in such a manner that the dots recorded by the use of the same nozzle are not continuous in the direction A. Therefore, even if there are the nozzles causing the displacement of the recording dots, the dots thus recorded do not appear continuously in the direction A. The advantage is that the white streaks in the direction A are not remarkably noticeable.
However, if there are any nozzles performing incomplete ejection, a problem is still encountered that the white streaks remain as clear image defects, although the white streaks are less conspicuous by the multi-scanning than by the usual serial scanning. Particularly when the recording duty is high, the ink mist is apt to be generated. This type of ink mist is accumulated on the head surface to cover the nozzles; thus disabling the ink ejection in some cases. A disabled ink ejection of the kind is different from the genuine nozzle clogging or the like, and is dependent on the degree of the density of an image. As a result, it occurs at random in an image or it is often recovered itself; thus making its countermeasure difficult.
Also, in FIG. 3B, when the head is scanned in the direction A to record an image, the head temperature is increased due to the accumulation of the ejection driving energy. Thus, the viscosity of ink is lowered and the ejecting amount of ink is also increased. As a result, the image density is in general higher toward the termination of recording than at the time of initiating the recording at each scan. This phenomenon generally presents a problem in any image, but particularly when output images are joined together to form one image, that is, when the so-called enlarged continuous copying mode is used, the difference in densities will become more conspicuous. If a multi-scanning is performed, the number of ink droplets ejected per unit time per nozzle is reduced as is clear from FIG. 3B. In the case represented in FIG. 3B, it is reduced to a ⅓ approximately. Therefore, the head temperature rise is suppressed as compared to the case of using the usual serial scanning. However, the above-mentioned problem still remains unsolved. Also, the seriousness of this problem differs depending on the image pattern to be recorded. In other words, when a pattern having a large image ratio is recorded, this becomes a serious problem, but when a pattern having a small image ratio is recorded, it is not so serious a problem.